Advances in the applications of current materials demand increased strength and toughness from existing monolithic materials. These requirements may be met by reinforcing the monolithic materials with high strength continuous fibres to produce composites. A composite material body comprises two components: the load bearing high strength fibre and a ductile load distributing matrix. Metallic matrices incorporating high strength ceramic fibres, in particular titanium or aluminum alloys reinforced with SiC fibres, result in composites whose properties can be engineered in terms of strength, stiffness, corrosion resistance, density and ductility. These improved material properties, compared to base monolithic materials, are utilized in different geometries such as flat, curved, circular and other shapes, based on the requirements of the particular structural component. The current fabrication of these composites involves cumbersome multi-step processes, predominantly intended for the manufacture of flat parts.
U.S. Pat. No. 3,615,277 describes a method of manufacturing fibre reinforced articles using monolayer composite tapes. The fibre material is attached to a mandrel and placed in front of a plasma spray. By rotating and translating the mandrel, a homogeneous matrix layer is deposited onto the fibres. On cooling, the tape containing the fibres/matrix monolayer is removed from the mandrel and cut to required shape.
U.S. Pat. No. 4,518,625 describes the use of an arc metal spray gun to spray hot molten metal onto previously aligned fibres on a large drum. The chamber containing the fibre wound drum is evacuated and back filled with argon to provide an inert and contaminant free environment. After spraying, the monolayer is removed and cut to the required dimensions.
Both of these processes are used to make flat components. A multi-layer flat part is produced by stacking several cut monolayers and compacting them by using external pressure and/or temperature. The fibres have to be aligned on the drum or mandrel using organic binders which must be removed through a burn-out cycle prior to densification, and may cause contamination due to incomplete removal. Use of high temperature and pressure for densification of the stacks can damage the fibres.
For making axi-symmetric components, the possible fabrication techniques include: pressing layers of matrix-fibre monolayers, wire winding and fibre coating techniques. In the first case, the process consists of pressing individual foil-fibre mats, where each monolayer is laid in the form of a flat matrix disc. Initially, grooves are etched in the circumferential direction by chemical etching or photolithography. This results in a grooved disc into which a single strand of the fibre is aligned circumferentially, with the help of organic binders. These fibre-matrix monolayers are then hot pressed to obtain a circular composite with the fibres arranged in the circumferential direction, again using high temperature and/or pressure for densification. In this process, the matrix to be used must be available in the form of a sheet. This is not always the case, especially with matrices of low ductility, such as certain aluminides of titanium, nickel or iron. Additionally, the carbonaceous char which may be left behind after the burn-out cycle may result in fabrication problems such as: a) formation of porosity, b) internal matrix oxidation during subsequent heat treatments, resulting in poor mechanical properties; and c) carbon from the char chemically combining with the matrix to form brittle carbides.
In the wire winding process, fibres and matrix in the form of wires are wound onto a mandrel. The winding mechanism requires precision equipment to ensure appropriate spacing and fibre distribution throughout the composite. The matrix wire diameter controls the volume fraction of the fibres. After a suitable thickness has been wound, the entire assembly is encased in a can which is evacuated to a low pressure and hot isostatically pressed (HIP-ed), resulting in an axi-symmetric component. For this process, the matrix material should be available in the form of wires, again limiting the scope of possible matrix materials. This method naturally excludes the use of matrices with lower ductility. Given the nature of winding, the porosity in the preform can be as high as 30%, which may lead to further fibre movement and possible damage during HIP-ing. The can material must be machined off after HIP-ing, thus adding to processing costs.
The third process involves the coating of the reinforcing fibre with the matrix alloy by a high speed deposition process such as the Electron Beam Deposition (EBD). Individual fibre strands are rotated above an alloy bath. The evaporation of the alloy may be achieved in a single step, if the vapour pressures of the alloy constituents are of the order of 1 torr, or by coevaporation of the constituent elements. Coated fibres are used to make a preform of an appropriate shape which is then consolidated by vacuum hot pressing or HIP-ing, to produce a 100% dense material. Similar to the wire winding process, high precision equipment is required to hold and rotate the fibre to ensure an even coating of the matrix alloy around the fibre.
None of the above mentioned techniques have the capability to produce an axi-symmetric component in a single step.
It is an object of the present invention to eliminate the above-mentioned problems associated with the existing fabrication techniques by developing a single step process for manufacturing axi-symmetric fiber-reinforced composite parts.